Apparatus for caries detection

ABSTRACT

An apparatus for obtaining an image of a tooth having at least one light source providing incident light having a first spectral range for obtaining a reflectance image from the tooth and a second spectral range for exciting a fluorescence image from the tooth. A polarizing beamsplitter in the path of the incident light from both sources directs light having a first polarization state toward the tooth and directs light from the tooth having a second polarization state along a return path toward a sensor, wherein the second polarization state is orthogonal to the first polarization state. A first lens in the return path directs image-bearing light from the tooth toward the sensor, and obtains image data from the portion of the light having the second polarization state. A long-pass filter in the return path attenuates light in the second spectral range.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Divisional of U.S. patent application Ser. No. 13/945,974,filed on Jul. 19, 2013, entitled APPARATUS FOR CARIES DETECTION, byLiang et al., the disclosure of which is incorporated herein, which wasitself a Divisional of U.S. patent application Ser. No. 13/485,996,filed on Jun. 1, 2012, entitled APPARATUS FOR CARIES DETECTION, by Lianget al., the disclosure of which is incorporated herein, which was itselfa Divisional of U.S. patent application Ser. No. 11/530,987, filed onSep. 12, 2006, entitled APPARATUS FOR CARIES DETECTION, by Liang et al.,which issued as U.S. Pat. No. 8,270,689, the disclosure of which isincorporated herein.

FIELD OF THE INVENTION

This invention generally relates to methods and apparatus for dentalimaging and more particularly relates to an apparatus for cariesdetection using fluorescence and scattering.

BACKGROUND OF THE INVENTION

In spite of improvements in detection, treatment, and preventiontechniques, dental caries remains a widely prevalent condition affectingpeople of all age groups. If not properly and promptly treated, cariescan lead to permanent tooth damage and even to loss of teeth.

Traditional methods for caries detection include visual examination andtactile probing with a sharp dental explorer device, often assisted byradiographic (x-ray) imaging. Detection using these methods can besomewhat subjective, varying in accuracy due to many factors, includingpractitioner expertise, location of the infected site, extent ofinfection, viewing conditions, accuracy of x-ray equipment andprocessing, and other factors. There are also hazards associated withconventional detection techniques, including the risk of damagingweakened teeth and spreading infection with tactile methods as well asexposure to x-ray radiation. By the time caries is evident under visualand tactile examination, the disease is generally in an advanced stage,requiring a filling and, if not timely treated, possibly leading totooth loss.

In response to the need for improved caries detection methods, there hasbeen considerable interest in improved imaging techniques that do notemploy x-rays. One method that has been commercialized employsfluorescence, caused when teeth are illuminated with high intensity bluelight. This technique, termed quantitative light-induced fluorescence(QLF), operates on the principle that sound, healthy tooth enamel yieldsa higher intensity of fluorescence under excitation from somewavelengths than does de-mineralized enamel that has been damaged bycaries infection. The strong correlation between mineral loss and lossof fluorescence for blue light excitation is then used to identify andassess carious areas of the tooth. A different relationship has beenfound for red light excitation, a region of the spectrum for whichbacteria and bacterial by-products in carious regions absorb andfluoresce more pronouncedly than do healthy areas.

Among proposed solutions for optical detection of caries are thefollowing:

U.S. Pat. No. 4,290,433 (Alfano) discloses a method to detect caries bycomparing the excited luminescence in two wavelengths.

U.S. Pat. No. 4,479,499 (Alfano) describes a method to detect caries bycomparing the intensity of the light scattered at two differentwavelengths.

U.S. Pat. No. 4,515,476 (Ingmar) discloses use of a laser for providingexcitation energy that generates fluorescence at some other wavelengthfor locating carious areas.

U.S. Pat. No. 6,231,338 (de Josselin de Jong et al.) discloses animaging apparatus for identifying dental caries using fluorescencedetection.

U.S. Patent Application No. 2004/0240716 (de Josselin de Jong et al.)discloses methods for improved image analysis for images obtained fromfluorescing tissue.

Among commercialized products for dental imaging using fluorescencebehavior is the QLF Clinical System from Inspektor Research Systems BV,Amsterdam, The Netherlands. Using a different approach, the DiagnodentLaser Caries Detection Aid from KaVo Dental GmbH, Biberach, Germany,detects caries activity monitoring the intensity of fluorescence ofbacterial by-products under illumination from red light.

U.S. Patent Application Publication 2005/0003323 (Katsuda et al.)describes a hand-held imaging apparatus suitable for medical or dentalapplications, using fluorescence imaging. The '3323 Katsuda et al.disclosure shows an apparatus that receives the reflection light fromthe diagnostic object and/or the fluorescence of the diagnostic objectwith different light irradiation. The disclosed apparatus is fairlycomplicated, requiring switchable filters in the probe, for example.While the apparatus disclosed in the Katsuda et al. '3323 patentapplication takes advantage of combining reflection light andfluorescence from the diagnostic object in the same optical path, theapparatus does not remove or minimize specular reflection. Any unwantedspecular reflection produces false positive results in reflectanceimaging. Moreover, with the various illumination embodiments disclosed,the illumination directed toward a tooth or other diagnostic object isnot uniform, since the light source is in close proximity to thediagnostic object.

U.S. Patent Application Publication 2004/0202356 (Stookey et al.)describes mathematical processing of spectral changes in fluorescence inorder to detect caries in different stages with improved accuracy.Acknowledging the difficulty of early detection when using spectralfluorescence measurements, the '2356 Stookey et al. disclosure describesapproaches for enhancing the spectral values obtained, effecting atransformation of the spectral data that is adapted to the spectralresponse of the camera that obtains the fluorescent image.

While the disclosed methods and apparatus show promise in providingnon-invasive, non-ionizing imaging methods for caries detection, thereis still room for improvement. One recognized drawback with existingtechniques that employ fluorescence imaging relates to image contrast.The image provided by fluorescence generation techniques such as QLF canbe difficult to assess due to relatively poor contrast between healthyand infected areas. As noted in the '2356 Stookey et al. disclosure,spectral and intensity changes for incipient caries can be very slight,making it difficult to differentiate non-diseased tooth surfaceirregularities from incipient caries.

Overall, it is well-recognized that, with fluorescence techniques, theimage contrast that is obtained corresponds to the severity of thecondition. Accurate identification of caries using these techniquesoften requires that the condition be at a more advanced stage, beyondincipient or early caries, because the difference in fluorescencebetween carious and sound tooth structure is very small for caries at anearly stage. In such cases, detection accuracy using fluorescencetechniques may not show marked improvement over conventional methods.Because of this shortcoming, the use of fluorescence effects appears tohave some practical limits that prevent accurate diagnosis of incipientcaries. As a result, a caries condition may continue undetected until itis more serious, requiring a filling, for example.

Detection of caries at very early stages is of particular interest forpreventive dentistry. As noted earlier, conventional techniquesgenerally fail to detect caries at a stage at which the condition can bereversed. As a general rule of thumb, incipient caries is a lesion thathas not penetrated substantially into the tooth enamel. Where such acaries lesion is identified before it threatens the dentin portion ofthe tooth, remineralization can often be accomplished, reversing theearly damage and preventing the need for a filling. More advancedcaries, however, grows increasingly more difficult to treat, most oftenrequiring some type of filling or other type of intervention.

In order to take advantage of opportunities for non-invasive dentaltechniques to forestall caries, it is necessary that caries be detectedat the onset. In many cases, as is acknowledged in the '2356 Stookey etal. disclosure, this level of detection has been found to be difficultto achieve using existing fluorescence imaging techniques, such as QLF.As a result, early caries can continue undetected, so that by the timepositive detection is obtained, the opportunity for reversal usinglow-cost preventive measures can be lost.

U.S. Pat. No. 6,522,407 (Everett et al.) discloses the application ofpolarimetry principles to dental imaging. One system described in theEverett et al. '407 teaching provides a first polarizer in theillumination path for directing a polarized light to the tooth. A secondpolarizer is provided in the path of reflected light. In one position,the polarizer transmits light of a horizontal polarization. Then, thepolarizer is oriented to transmit light having an orthogonalpolarization. Intensity of these two polarization states of thereflected light can then be compared to calculate the degree ofdepolarization of light scattered from the tooth. The result of thiscomparison then provides information on a detected caries infection.

While the approach disclosed in the Everett et al. '407 patent takesadvantage of polarization differences that can result frombackscattering of light, the apparatus and methods described thereinrequire the use of multiple polarizers, one in the illumination path,the other in the imaging path. Moreover, the imaging path polarizer mustsomehow be readily switchable between a reference polarization state andits orthogonal polarization state. Thus, this solution has inherentdisadvantages for allowing a reduced package size for caries detectionoptics. It would be advantageous to provide a simpler solution forcaries imaging, a solution not concerned with measuring a degree ofdepolarization, thus using a smaller number of components and notrequiring switchable orientation of a polarizer between one of twopositions.

As is described in one embodiment of the Everett et al. '407 patentdisclosure, optical coherence tomography (OCT) has been proposed as atool for dental and periodontal imaging, as well as for other medicalimaging applications. For example:

U.S. Pat. No. 5,321,501 (Swanson et al.) describes principles of OCTscanning and measurement as used in medical imaging applications;

U.S. Pat. No. 5,570,182 (Nathel et al.) describes the use of OCT forimaging of tooth and gum structures;

U.S. Pat. No. 6,179,611 (Everett et al.) describes a dental explorertool that is configured to provide a scanned OCT image;

U.S. Patent Application Publication No. 2005/0024646 (Quadling et al.)describes the use of time-domain and Fourier-domain OCT systems fordental imaging;

Japanese Patent Application Publication No. JP 2004-344260 (Kunitoshi etal.) discloses an optical diagnostic apparatus equipped with a camerafor visual observation of a tooth part, with visible light forilluminating a surface image, and an OCT device for scanning theindicated region of a surface image using an alternate light source.

While OCT solutions, such as those described above, can provide verydetailed imaging of structure beneath the surface of a tooth, OCTimaging itself can be time-consuming and computation-intensive. OCTimages would be most valuable if obtained within one or more localregions of interest, rather than obtained over widespread areas. Thatis, once a dental professional identifies a specific area of interest,then OCT imaging could be provided for that particular area only.Conventional solutions, however, have not combined visible light imagingwith OCT imaging in the same imaging apparatus.

Thus, it can be seen that there is a need for a non-invasive,non-ionizing imaging method for caries detection that offers improvedaccuracy for detection of caries, particularly in its earlier stages,with a reduced number of components and reduced complexity overconventional solutions.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for imaging a toothcomprising: (a) at least one light source providing incident lighthaving a first spectral range for obtaining a reflectance image on thetooth and a second spectral range for exciting a fluorescence image ofthe tooth; (b) a polarizing beamsplitter in a path of the incidentlight, the polarizing beamsplitter directing light having a firstpolarization state toward the tooth and directing light from the toothhaving a second polarization state along a return path toward a sensor,wherein the second polarization state is orthogonal to the firstpolarization state; (c) a lens positioned in the return path to directimage-bearing light from the tooth toward the sensor for obtaining imagedata from the portion of the light having the second polarization state;and (d) a long-pass filter in the return path, to attenuate light in thesecond spectral range and to transmit light in the first spectral range.

It is a feature of the present invention that it utilizes bothfluorescence and reflectance image data for dental imaging.

It is an advantage of the present invention that it offers enhancementover existing fluorescence imaging techniques, useful for detection ofcaries in its incipient stages.

These and other objects, features, and advantages of the presentinvention will become apparent to those skilled in the art upon areading of the following detailed description when taken in conjunctionwith the drawings wherein there is shown and described an illustrativeembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description when taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a schematic block diagram of an imaging apparatus for cariesdetection according to one embodiment;

FIG. 2 is a schematic block diagram of an imaging apparatus for cariesdetection according to an alternate embodiment;

FIG. 3 is a schematic block diagram of an imaging apparatus for cariesdetection according to an alternate embodiment;

FIG. 4A is a schematic block diagram of an imaging apparatus for cariesdetection according to an alternate embodiment using polarized light;

FIG. 4B is a schematic block diagram of an imaging apparatus for cariesdetection according to an alternate embodiment using a polarizingbeamsplitter to provide polarized light;

FIG. 4C is a schematic block diagram of an alternate embodiment using aband pass filter with a narrow band light source;

FIG. 5 is a view showing the process for combining dental image data togenerate a fluorescence image with reflectance enhancement according tothe present invention;

FIG. 6 is a composite view showing the contrast improvement of thepresent invention in a side-by-side comparison with conventional visualand fluorescence methods;

FIG. 7 is a block diagram showing a sequence of image processing forgenerating an enhanced threshold image according to one embodiment;

FIG. 8 is a schematic block diagram of an imaging apparatus for cariesdetection according to an alternate embodiment using multiple lightsources;

FIG. 9 is a schematic block diagram of an imaging apparatus for cariesdetection using polarized light in one embodiment of the presentinvention;

FIG. 10 is a schematic block diagram of an imaging apparatus for cariesdetection using polarized light in an alternate embodiment of thepresent invention;

FIG. 11 is a schematic block diagram of an imaging apparatus for cariesdetection using polarized light in an alternate embodiment of thepresent invention;

FIG. 12A is a schematic block diagram of an imaging apparatus for cariesdetection using polarized light from two sources in an alternateembodiment of the present invention;

FIG. 12B is a schematic block diagram of an imaging apparatus for cariesdetection using a ring illuminator with LEDs in an alternate embodimentof the present invention;

FIG. 12C is a schematic block diagram of an imaging apparatus for cariesdetection using a fiber ring illuminator in an alternate embodiment ofthe present invention;

FIG. 13 is a schematic block diagram of an imaging apparatus for cariesdetection using polarized light and OCT scanning in one embodiment;

FIG. 14A is a plan view of an operator interface screen in oneembodiment;

FIG. 14B is an example display of OCT scanning results;

FIG. 15 is a block diagram showing an arrangement of a hand-held imagingapparatus in one embodiment;

FIG. 16 is a perspective view showing an imaging apparatus having anintegral display;

FIG. 17 is a block diagram showing combination of multiple types ofimages in order to form a composite image;

FIG. 18 is a block diagram showing a wireless dental imaging system inone embodiment;

FIG. 19 is a block diagram of an alternate embodiment for the imagingprobe with two sensors;

FIG. 20 is a logic flow diagram for image processing workflow;

FIG. 21 is a block diagram showing an image relay arrangement used inone embodiment;

FIG. 22 is a block diagram showing the path of emitted light within theapparatus of the present invention;

FIGS. 23A and 23B are block diagrams of embodiments for image capturewith auto-focusing capability;

FIG. 23C is a diagram showing how focusing indicators operate;

FIG. 24 is a graph showing characteristic curves for white light and fora long pass filter used in the apparatus of the present invention; and

FIG. 25 is a diagram showing operation of a toggle switch for obtainingseparate images.

DETAILED DESCRIPTION OF THE INVENTION

The present description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance withthe invention. It is to be understood that elements not specificallyshown or described may take various forms well known to those skilled inthe art.

Reference is made to U.S. application Ser. No. 11/262,869, filed Oct.31, 2005, entitled METHOD FOR DETECTION OF CARIES, by Wong et al, whichissued as U.S. Pat. No. 7,596,253, incorporated herein by reference inits entirety.

Reference is made to U.S. application Ser. No. 11/408,360, filed Apr.21, 2006, entitled OPTICAL DETECTION OF DENTAL CARIES by Wong et al,which issued as U.S. Pat. No. 7,577,284, incorporated herein byreference in its entirety.

Reference is made to U.S. patent application Ser. No. 11/530,913, filedSep. 12, 2006, entitled LOW COHERENCE DENTAL OCT IMAGING, by Liang etal, which published as US 2008/0062429, incorporated herein by referencein its entirety.

As noted in the preceding background section, it is known thatfluorescence can be used to detect dental caries using either of twocharacteristic responses: First, excitation by a blue light sourcecauses healthy tooth tissue to fluoresce in the green spectrum.Secondly, excitation by a red light source can cause bacterialby-products, such as those indicating caries, to fluoresce in the redspectrum.

In order for an understanding of how light is used in the presentinvention, it is important to give more precise definition to the terms“reflectance” and “back-scattering” as they are used in biomedicalapplications in general and, more particularly, in the method andapparatus of the present invention. In broadest optical parlance,reflectance generally denotes the sum total of both specular reflectanceand scattered reflectance. (Specular reflection is that component of theexcitation light that is reflected by the tooth surface at the sameangle as the incident angle.) In biomedical applications, however, as inthe dental application of the present invention, the specular componentof reflectance is of no interest and is, instead, generally detrimentalto obtaining an image or measurement from a sample. The component ofreflectance that is of interest for the present application is fromback-scattered light only. Specular reflectance must be blocked orotherwise removed from the imaging path. With this distinction in mind,the term “back-scattered reflectance” is used in the present applicationto denote the component of reflectance that is of interest.“Back-scattered reflectance” is defined as that component of theexcitation light that is elastically back-scattered over a wide range ofangles by the illuminated tooth structure. “Reflectance image” data, asthis term is used in the present invention, refers to image dataobtained from back-scattered reflectance only, since specularreflectance is blocked or kept to a minimum. In the scientificliterature, back-scattered reflectance may also be referred to asback-reflectance or simply as backscattering. Back-scattered reflectanceis at the same wavelength as the excitation light.

It has been shown that light scattering properties differ between soundand carious dental regions. In particular, reflectance of light from theilluminated area can be at measurably different levels for normal versuscarious areas. This change in reflectance, taken alone, may not besufficiently pronounced to be of diagnostic value when considered byitself, since this effect is very slight, although detectable. For moreadvanced stages of caries, for example, back-scattered reflectance maybe less effective an indicator than at earlier stages.

In conventional fluorescence measurements such as those obtained usingQLF techniques, reflectance itself is an effect that is avoided ratherthan utilized. A filter is usually employed to block off all excitationlight from reaching the detection device. For this reason, the slightbut perceptible change in back-scattered reflectance from excitationlight has received little attention for diagnosing caries.

The inventors have found, however, that this back-scattered reflectancechange can be used in conjunction with fluorescence effects to moreclearly and more accurately pinpoint a carious location. Moreover, theinventors have observed that the change in light scattering activity,while it can generally be detected wherever a caries condition exists,is more pronounced in areas of incipient caries. This back-scatteredreflectance change is evident at early stages of caries, even whenfluorescent effects are least pronounced.

The present invention takes advantage of the observed back-scatteringbehavior for incipient caries and uses this effect, in combination withfluorescence effects described previously in the background section, toprovide an improved capability for dental imaging to detect caries. Theinventive technique, hereafter referred to as fluorescence imaging withreflectance enhancement (FIRE), not only helps to increase the contrastof images over that of earlier approaches, but also makes it possible todetect incipient caries at stages where preventive measures are likelyto effect remineralization, repairing damage done by the cariesinfection at a stage well before more complex restorative measures arenecessary. Advantageously, FIRE detection can be accurate at an earlierstage of caries infection than has been exhibited using existingfluorescence approaches that measure fluorescence alone.

Imaging Apparatus

Referring to FIG. 1, there is shown one basic optical arrangement for animaging apparatus 10 for caries detection using the FIRE method in oneembodiment. A light source 12 directs an incident light, at a bluewavelength range or other suitable wavelength range, toward tooth 20through an optional lens 14 or other light beam conditioning component.The tooth 20 may be illuminated at a smooth surface (as shown) or at anocclusal surface (not shown). Two components of light are then detectedby a monochrome camera 30 through a lens 22: a back-scattered lightcomponent having the same wavelength as the incident light and havingmeasurable reflectance; and a fluorescent light that has been exciteddue to the incident light. For FIRE imaging, specular reflection causesfalse positives and is undesirable. To minimize specular reflection pickup, the camera 30 is positioned at a suitable angle with respect to thelight source 12. This allows imaging of back-scattered light without theconfounding influence of a specularly reflected component.

In the embodiment of FIG. 1, monochrome camera 30 has color filters 26and 28. One of color filters 26 or 28 is used during reflectanceimaging; the other is used during fluorescence imaging. A processingapparatus 38 obtains and processes the reflectance and fluorescenceimage data and forms a FIRE image 60. FIRE image 60 is an enhanceddiagnostic image that can be printed or can appear on a display 40. FIREimage 60 data can also be transmitted to storage or transmitted toanother site for display.

Referring to FIG. 2, there is shown the basic optics arrangement in analternate embodiment using a color camera 32. With this arrangement,auxiliary filters would not generally be needed, since color camera 32would be able to obtain the reflectance and fluorescence images from thecolor separations of the full color image of tooth 20.

Light source 12 is typically centered around a blue wavelength, such asabout 405 nm in one embodiment. In practice, light source 12 could emitlight ranging in wavelength from an upper ultraviolet range to blue,between about 300 and 500 nm. Light source 12 can be a laser or could befabricated using one or more light emitting diodes (LEDs). Alternately,a broadband source, such as a xenon lamp, having a supporting colorfilter for passing the desired wavelengths could be used. Lens 14 orother optical elements may serve to condition the incident light, suchas by controlling the uniformity and size of the illumination area. Forexample, a diffuser 13, shown as a dotted line in FIG. 2, might be usedbefore or after lens 14 to smooth out the hot spots of an LED beam. Thepath of illumination light might include light guiding or lightdistributing structures such as an optical fiber or a liquid lightguide, for example (not shown). Light level is typically a fewmilliwatts in intensity, but can be more or less, depending on the lightconditioning and sensing components used.

Referring to the basic optical arrangement shown in FIG. 3, illuminationcomponents could alternately direct light at normal incidence, turnedthrough a beamsplitter 34. Camera 32 would then be disposed to obtainthe image light that is transmitted through beamsplitter 34. Otheroptions for illumination include multiple light sources directed at thetooth with angular incidence from one or more sides. Alternately, theillumination might use an annular ring or an arrangement of LED sourcesdistributed about a center such as in a circular array to provide lightuniformly from multiple angles as shown in FIGS. 12A and 12B.Illumination could also be provided through an optical fiber or fiberarray as shown in FIG. 12C.

The imaging optics, represented as lens 22 in FIGS. 1-3, could includeany suitable arrangement of optical components, with possibleconfigurations ranging from a single lens component to a multi-elementlens. Clear imaging of the tooth surface, which is not flat but can haveareas that are both smoothly contoured and highly ridged, requires thatimaging optics have sufficient depth of field. Preferably, for optimalresolution, the imaging optics provides an image size that substantiallyfills the sensor element of the camera.

Image capture can be performed by either monochrome camera 30 (FIG. 1)or color camera 32 (FIG. 2). Typically, camera 30 or 32 employs a CMOSor CCD sensor. The monochrome version would typically employ aretractable spectral filter 26, 28 suitable for the wavelength ofinterest. For light source 12 having a blue wavelength, spectral filter26 for capturing reflectance image data would transmit predominatelyblue light. Spectral filter 28 for capturing fluorescence image datawould transmit light at a different wavelength, such as predominatelygreen light. Preferably, spectral filters 26 and 28 are automaticallyswitched into place to allow capture of both reflectance andfluorescence images in very close succession. Both images are obtainedfrom the same position to allow accurate registration of the image data.

Spectral filter 28 would be optimized with a pass-band that capturesfluorescence data over a range of suitable wavelengths. The fluorescenteffect that has been obtained from tooth 20 can have a relative broadspectral distribution in the visible range, with light emitted that isoutside the wavelength range of the light used for excitation. Thefluorescent emission is typically between about 450 nm and 600 nm, whilegenerally peaking in the green region, roughly from around 510 nm toabout 550 nm. Thus a green light filter is generally preferred forspectral filter 28 in order to obtain this fluorescence image at itshighest energy levels. With color camera 32, the green image data isgenerally used for this same reason. This green image data is alsoobtained through a green light filter, such as a green filter in a colorfilter array (CFA), as is well known to those skilled in the color imagecapture art. However, other ranges of the visible spectrum could also beused in other embodiments.

Camera controls are suitably adjusted for obtaining each type of image.For example, when capturing the fluorescence image, it is necessary tomake appropriate exposure adjustments for gain, shutter speed, andaperture, since this image may not be intense. When using color camera32 (FIG. 2), color filtering is performed by the color filter arrays onthe camera image sensor. The reflectance image is captured in the bluecolor plane; simultaneously, the fluorescence image is captured in thegreen color plane. That is, a single exposure captures bothback-scattered reflectance and fluorescence images.

Processing apparatus 38 is typically a computer workstation but may, inits broadest application, be any type of control logic processingcomponent or system that is capable of obtaining image data from camera30 or 32 and executing image processing algorithms upon that data togenerate the FIRE image 60 data. Processing apparatus 38 may be local ormay connect to image sensing components over a networked interface.

Referring to FIG. 5, there is shown, in schematic form, how the FIREimage 60 is formed according to the present invention. Two images oftooth 20 are obtained, a green fluorescence image 50 and a bluereflectance image 52. As noted earlier, it must be emphasized that thereflectance light used for reflectance image 52 and its data is fromback-scattered reflectance, with specular reflectance blocked or kept aslow as possible. In the example of FIG. 5, there is a carious region 58,represented in phantom outline in each of images 50, 52, and 60, whichcauses a slight decrease in fluorescence and a slight increase inreflectance. The carious region 58 may be imperceptible or barelyperceptible in either fluorescence image 50 or reflectance image 52,taken individually. Processing apparatus 38 operates upon the image datausing an image processing algorithm as discussed below for both images50 and 52 and provides FIRE image 60 as a result. The contrast betweencarious region 58 and sound tooth structure is heightened, so that acaries condition is made more visible in FIRE image 60.

FIG. 6 shows the contrast improvement of the present invention in aside-by-side comparison with a visual white-light image 54 andconventional fluorescence methods. For caries at a very early stage, thecarious region 58 may look indistinct from the surrounding healthy toothstructure in white-light image 54, either as perceived directly by eyeor as captured by an intraoral camera. In the green fluorescence image52 captured by existing fluorescence method, the carious region 58 mayshow up as a very faint, hardly noticeable shadow. In contrast, in theFIRE image 60 generated by the present invention, the same cariousregion 58 shows up as a darker, more detectable spot. Clearly, the FIREimage 60, with its contrast enhancement, offers greater diagnosticvalue.

Image Processing

As described earlier with reference to FIGS. 5 and 6, processing of theimage data uses both the reflectance and fluorescence image data togenerate a final image that can be used to identify carious areas of thetooth. There are a number of alternative processing methods forcombining the reflectance and fluorescence image data to form FIRE image60 for diagnosis. Commonly-assigned U.S. patent application Ser. No.11/262,869, cited above, describes one method for combining reflectanceand fluorescence image data, using scalar multipliers and finding adifference between scaled reflectance and fluorescence values.

Following an initial combination of fluorescence and reflectance values,additional image processing may also be of benefit. A thresholdingoperation, executed using image processing techniques familiar to thoseskilled in the imaging arts, or some other suitable conditioning of thecombined image data used for FIRE image 60, may be used to furtherenhance the contrast between a carious region and sound tooth structure.Referring to FIG. 7, there is shown, in block diagram form, a sequenceof image processing for generating an enhanced threshold FIRE image 64according to one embodiment. Fluorescence image 50 and reflectance image52 are first combined to form FIRE image 60, as described previously. Athresholding operation is next performed, providing threshold image 62that defines more clearly the area of interest, carious region 58. Then,threshold image 62 is combined with original FIRE image 60 to generateenhanced threshold FIRE image 64. Similarly, the results of thresholddetection can also be superimposed onto a white light image 54 (FIG. 6)in order to definitively outline the location of a carious infection.

It can be readily appreciated that any number of complex imageprocessing algorithms could alternately be used for combining thereflectance and fluorescence image data in order to obtain an enhancedimage that identifies carious regions more clearly. It may beadvantageous to apply a number of different imaging algorithms to theimage data in order to obtain the most useful result. In one embodiment,an operator can elect to use any of a set of different image processingalgorithms for conditioning the fluorescence and reflectance image dataobtained. This would allow the operator to check the image data whenprocessed in a number of different ways and may be helpful foroptimizing the detection of carious lesions having differentshape-related characteristics or that occur over different areas of thetooth surface.

It is emphasized that the image contrast enhancement achieved in thepresent invention, because it employs both reflectance and fluorescencedata, is advantaged over conventional methods that use fluorescent imagedata only. Conventionally, where only fluorescence data is obtained,image processing has been employed to optimize the data, such as totransform fluorescence data based on spectral response of the camera orof camera filters or other suitable characteristics. For example, themethod of the '2356 Stookey et al. disclosure, cited above, performsthis type of optimization, transforming fluorescence image data based oncamera response. However, these conventional approaches overlook theadded advantage of additional image information that the back-scatteredreflectance data obtains.

Alternate Embodiments

The method of the present invention admits a number of alternateembodiments. For example, the contrast of either or both of thereflectance and fluorescence images may be improved by the use of apolarizing element. It has been observed that enamel, having a highlystructured composition, is sensitive to the polarization of incidentlight. Polarized light has been used to improve the sensitivity ofdental imaging techniques, for example, in “Imaging Caries Lesions andLesion Progression with Polarization Sensitive Optical CoherenceTomography” in J. Biomed Opt., 2002 October; 7(4): pp. 618-627, by Friedet al.

Specular reflection tends to preserve the polarization state of theincident light. For example, where the incident light is s-polarized,the specular reflected light is also s-polarized. Backscattering, on theother hand, tends to de-polarize or randomize the polarization of theincident light. Where incident light is s-polarized, back-scatteredlight has both s- and p-polarization components. Using a polarizer andanalyzer, this difference in polarization handling can be employed tohelp eliminate unwanted specular reflectance from the reflectance image,so that only back-scattered reflectance is obtained.

Referring to FIG. 4A, there is shown an embodiment of imaging apparatus10 that expands upon the basic model shown in FIGS. 1-3, employing apolarizer 42 in the path of the incident illumination light and othersupporting optics. Polarizer 42 transmits linearly polarized incidentlight. An optional analyzer 44 may also be provided in the return pathof image-bearing light from tooth 20 as a means to minimize the specularreflectance component. With this polarizer 42/analyzer 44 combination aspolarizing elements, reflectance light in the return path and sensed bycamera 30 or 32 is predominantly back-scattered light, that portion ofthe reflectance that is desirable for combination with the fluorescenceimage data according to the present invention. A long-pass filter 15 inthe path of returned light from the tooth is used to attenuateultraviolet and shorter wavelength visible light (for example, lightover the blue portion of the spectrum, centered near about 405+/−40 nm)and to pass longer wavelength light. This arrangement minimizes theeffect of blue light that may be used to excite fluorescence (normallycentered in the green portion of the spectrum, nominally about 550 nm)and, by attenuating this shorter-wavelength light, allows the use of awhite light source as light source 12 for obtaining a reflectance image.The curves of FIG. 24 show the overall relationship between a whitelight curve 98 (shown with a dashed line) and a long-pass filter curve96.

FIG. 4C shows an alternate embodiment using multiple light sources 12,each light source 12 having a different spectral range. Here, one lightsource 12 is a white light source for obtaining the reflectance image.The typical spectral range for a white light source can includewavelengths from about 400 to about 700 nm. The other light source 12 isa UV LED or other source that emits light having shorter wavelengths forexciting fluorescent emission. For example, its spectral range may bewell within 300-500 nm. A band pass filter 17 can be used to narrow theband and reduce optical crosstalk from this second light source into thefluorescence image.

Where there are multiple light sources 12, individual light sources 12can be toggled on and off in order to obtain the correspondingreflectance or fluorescence image at any one time. For the embodimentdescribed with reference to FIG. 4C, for example, white light source 12is on to obtain the reflectance image (or white light image) at camera32 or other sensor. The other UV LED source is off. Then, when whitelight source 12 is turned off and the UV LED source is energized, afluorescence image can be obtained.

FIG. 25 shows an embodiment with an imaging probe 104 having a toggleswitch 83 and the corresponding display 40 for each position of toggleswitch 83. In one position, as shown in the upper portion of FIG. 25,toggle switch 83 enables capture of fluorescence image 120. In anotherposition, shown in the lower portion of FIG. 25, toggle switch 83enables capture of reflectance image 122.

In an alternate embodiment, toggling in this fashion can be accomplishedautomatically, such as by control logic circuitry in communication withcamera 32 or sensor in imaging apparatus 10. This arrangement allows asingle camera 32 or other sensor to obtain images of different types.

An alternate embodiment, shown in FIG. 4B, employs a polarizingbeamsplitter 18 (sometimes termed a polarization beamsplitter) as apolarizing element. In this arrangement, polarizing beamsplitter 18advantageously performs the functions of both the polarizer and theanalyzer for image-bearing light, thus offering a more compact solution.Tracing the path of illumination and image-bearing light shows howpolarizing beamsplitter 18 performs this function. Illumination fromlight source 12 is essentially unpolarized. Polarizing beamsplitter 18transmits p-polarization, as shown by the dotted arrow in FIG. 4B, andreflects s-polarization, directing this light to tooth 20. At the tooth20, back-scattering depolarizes this light. Polarizing beamsplitter 18treats the back-scattered light in the same manner, transmitting thep-polarization and reflecting the s-polarization. The resultingp-polarized light can then be filtered at long-pass filter 15, anddetected at camera 30 (with suitable color filter as was described withreference to FIG. 1) or color camera 32. Because specular reflectedlight is s-polarized, polarizing beamsplitter 18 effectively removesthis specular reflective component from the light that reaches camera30, 32.

Polarized illumination results in further improvement in image contrast,but at the expense of light level, as can be seen from the descriptionof FIGS. 4A and 4B. Hence, when using polarized light in this way, itmay be necessary to employ a higher intensity light source 12. Thisemployment of polarized illumination is particularly advantaged forobtaining the reflectance image data and is also advantaged whenobtaining the fluorescence image data, increasing image contrast andminimizing the effects of specular reflection.

One type of polarizer 42 that has particular advantages for use inimaging apparatus 10 is the wire grid polarizer, such as those availablefrom Moxtek Inc. of Orem, Utah and described in U.S. Pat. No. 6,122,103(Perkins et al.) The wire grid polarizer exhibits good angular and colorresponse, with relatively good transmission over the blue spectralrange. Either or both polarizer 42 and analyzer 44 in the configurationof FIG. 4A could be wire grid polarizers. Wire grid polarizingbeamsplitters are also available, and can be used in the configurationof FIG. 4B.

The method of the present invention takes advantage of the way the toothtissue responds to incident light of sufficient intensity, using thecombination of fluorescence and light reflectance to indicate cariousareas of the tooth with improved accuracy and clarity. In this way, thepresent invention offers an improvement upon existing non-invasivefluorescence detection techniques for caries. As was described in thebackground section given above, images that have been obtained usingfluorescence only may not clearly show caries due to low contrast. Themethod of the present invention provides images having improved contrastand is, therefore, of more potential benefit to the diagnostician foridentifying caries.

In addition, unlike earlier approaches using fluorescence alone, themethod of the present invention also provides images that can be used todetect caries in its very early incipient stages. This added capability,made possible because of the perceptible back-scattering effects forvery early carious lesions, extends the usefulness of the fluorescencetechnique and helps in detecting caries during its reversible stages, sothat fillings or other restorative strategies might not be needed.

Referring to FIG. 9, there is shown an embodiment of imaging apparatus10 using polarized light from a polarizing beamsplitter 18 and using atelecentric field lens 24. Light source 12, typically a light source inthe blue wavelength range for exciting maximum fluorescence from tooth20 provides illumination through lens 14 and onto polarizingbeamsplitter 18. Here, one polarization state transmits, the other isreflected. In a typical embodiment, p-polarized light is transmittedthrough polarizing beamsplitter 18 and is, therefore, discarded. Thes-polarized light is reflected toward tooth 20, guided by field lens 24and an optional turning mirror 46 or other reflective surface. Lightreturning from tooth 20 can include a specular reflection component anda back-scattered reflection component. Specular reflectance does notchange the polarization state. Thus, for the s-polarized illumination,that is, for the unwanted specularly reflected component, the reflectedlight is directed back toward light source 12. As has been observed,back-scattered reflectance undergoes some amount of depolarization.Thus, some of the back-scattered reflected light has p-polarization andis transmitted through polarizing beamsplitter 18. This returning lightmay be further conditioned by optional analyzer 44 and then directed byan imaging lens 66 to sensor 68, such as a camera, through color filter56. Long pass filtering (not shown in FIG. 9) could also be employed inthe path of light returned from tooth 20. Color filter 56 is used inthis arrangement to block light from the light source that was used toexcite fluorescence, since the response of the color filter array (CFA)built inside the sensor is typically not sharp enough to block the lightfrom the light source in this region. In this way, the returning lightdirected to sensor 68 is fluorescence only.

The use of telecentric field lens 24 is advantaged in the embodiment ofFIG. 9. Telecentric optics provides constant magnification within thedepth of field, which is particularly useful for highly contouredstructures such as teeth that are imaged at a short distance.Perspective distortion is minimized. Telecentric field lens 24 could bea multi-element lens, represented by a single lens symbol in FIG. 9.Light source 12 may be any suitable color, including white, blue, green,red, or near infrared, for example. Light source 12 may also be a morecomplex assembly capable of providing light at different spectral bands,such as through the use of movable color filters. FIG. 10 shows analternate embodiment of imaging apparatus 10 in which no turning mirroris used. Instead, polarizing beamsplitter 18 is disposed in the imagingpath between field lens 24 and tooth 20. Alternately, if no field lensis used, polarizing beamsplitter 18 is disposed in the imaging path justbefore tooth 20. Light source 12 is positioned to direct illuminationthrough polarizing beamsplitter 18, so that the illumination effectivelybypasses field lens 24 if any. Specularly reflected light is againdiscarded by means of polarizing beamsplitter 18 and analyzer 44.

The block diagram of FIG. 11 shows an alternate embodiment of imagingapparatus 10 in which two separate light sources 12 a and 12 b are used.Light sources 12 a and 12 b may both emit the same wavelengths or mayemit different wavelengths. They may illuminate tooth 20 simultaneouslyor one at a time. Polarizing beamsplitter 18 is disposed in the imagingpath between field lens 24 and tooth 20, thus providing both turning andpolarization functions.

FIG. 12A shows another alternate embodiment, similar to that shown inFIG. 11, in which each of light sources 12 a and 12 b has acorresponding polarizer 42 a and 42 b. A turning mirror could besubstituted for polarizing beamsplitter 18 in this embodiment; however,the use of both polarized illumination, as provided from the combinationof light sources 12 a and 12 b and their corresponding polarizers 42 aand 42 b, and polarizing beamsplitter 18 can be advantageous forimproving image quality. FIG. 12B is another embodiment with additionalLEDs to increase the light level on the tooth or other object. Asdescribed above, the LEDs can be white light LEDs and/or blue LEDs. Inorder to achieve uniform illumination, the arrangement of LEDs, withrespect to the tooth or other object, should be symmetric.

FIG. 12C is another embodiment with an alternate illuminationimplementation. In this embodiment, fiber bundles are used to directlight from LEDs or other light source to the tooth or other object. InFIG. 12C, four optical fiber bundles 49 a-49 d are used. Fiber bundles49 a and 49 b are used to deliver white light to the object. Twopolarizers 42 a and 42 b are placed in front of the output surface ofoptical fiber bundles 49 b, 49 d to create linear polarizedillumination. Fiber bundles 49 c and 49 d couple the light from Blue orUV LEDs or other light sources to excite the fluorescence from theobject.

FIG. 19 is an alternate embodiment for the imaging probe with twosensors 68 a and 68 b. One dichroic mirror 48 is used as a spectralseparator in this embodiment to direct the reflected light withdifferent spectral bands to two different sensors. For example, in oneembodiment, dichroic mirror 48 transmits light within the visiblespectrum (440 nm to 650 nm) and reflects UV (<400 nm) and NIR (>700 nm).With this embodiment, the imaging probe can also be employed in otherapplications, such as for tooth color shade matching and for soft tissueimaging, for example.

In one embodiment, the apparatus displays a fluorescence image, insteadof a white light image, as the live video image. This enables theoperator to screen the tooth for caries detection using fluorescenceimaging and to assess tooth condition using the white light image forother applications. One switch is necessary, either in the probe or inthe software, so that the user can select the live video image modebased on the application. The switch in the probe can be a two-stepbutton switch. Without pressing the button switch, the live video imageis a white light image. When the button is pressed to its half waytravel position, the fluorescence image becomes the live video image.Both fluorescence and white light images can be captured and saved whenthe button is pressed to its full travel position.

With high-speed electronics and software, two live video images can bedisplayed in the monitor to the user. Both single-sensor and dual-sensorconfigurations can be used to display two live video images. In order toobtain two live images and avoid crosstalk, the LEDs with differentwavelengths need to be switched alternately on and off. An advantage indisplaying two live video images is that the user can compare thefluorescence and white light image and diagnose suspicious regions ofthese images. Using image processing utilities described subsequently, asuspicious region can be highlighted automatically when the fluorescenceand white image are alternately obtained.

One common difficulty with conventional intra-oral cameras and cariesdetection image devices is that the live video image moves in thedirection opposite to probe movement. This is due to imaging lensproperties: the image is reversed when using only one imaging lens.Several optical methods can be applied to form the image. One of themethods is to use an image relay technique, as shown in FIG. 21. Imagelens 222 forms an intermediate image 224 of the object, tooth 20. Theorientation of intermediate image 224 is opposite to the object 220. Animage lens 226 then forms a final image 228 of intermediate image 224.The orientation of image 228 is the same as the object, tooth 20. Usingthis embodiment, the moving direction of the final image 228 will be thesame as the probe movement. In addition, folding mirrors can be used tochange image orientation as necessary. Even without such extra opticalcomponents to change image orientation, software can manipulate theimage to correct the image orientation that is displayed to the user.

FIGS. 23A and 23B are two embodiments for image capture withauto-focusing capability. For simplicity, white light LEDs and LEDs forfluorescence imaging are not shown in FIGS. 23A and 23B. Light sources250 a and 250 b are LEDs with integral lenses. Collimating lenses inlight sources 250 a and 250 b form images 252 a and 252 b, respectively,onto a cross point 256 of the object plane and optical axis. As shown inFIG. 23A, when the probe is not in the right position (indicatingfocus), images 252 a and 252 b do not overlap. FIG. 23C shows how thisis implemented in one embodiment, with tooth 20 at different positionsrelative to cross point 256. At left in this figure, tooth 20 is beyondcross point 256, thus out of focus. At right, tooth 20 is within crosspoint 256, also out of focus. In the central portion of this figure,tooth 20 is positioned within focus. To achieve focus with thisarrangement, the operator simply moves the probe so that images 252 aand 252 b overlap. When images 252 a and 252 b overlap, the user caninstruct the system to take the images. As an alternative embodiment,auto focus can be provided. Software working with or within controlcircuitry 110 can detect and track the positions of images 252 a and 252b, using light detection techniques familiar to those skilled in theimaging arts. The software can then trigger sensor 68 or the camera totake the images once images 252 a and 252 b overlap.

FIG. 23B is a simplified version of FIG. 23A. In this configuration,only one LED and lens 250 a is used. A crosshair 254 displays on themonitor to indicate the center of the image and the optical axis. Whenimages 252 a align with crosshair 254, the probe is on focus. Ifsoftware is applied to track the position of image 252 a, the use ofcrosshair 254 or similar feature is not required.

Embodiments Using Optical Coherence Tomography (OCT)

Optical coherence tomography (OCT) is a non-invasive imaging techniquethat employs interferometric principles to obtain high resolution,cross-sectional tomographic images of internal microstructures of thetooth and other tissue that cannot be obtained using conventionalimaging techniques. Due to differences in the backscattering fromcarious and healthy dental enamel OCT can determine the depth ofpenetration of the caries into the tooth and determine if it has reachedthe dentin enamel junction. From area OCT data it is possible toquantify the size, shape, depth, and determine the volume of cariousregions in a tooth.

In an OCT imaging system for living tissue, light from a low-coherencesource, such as an LED or other light source, can be used. This light isdirected down two different optical paths: a reference arm of knownlength and sample arm, which goes to the tooth. Reflected light fromboth reference and sample arms is then recombined, and interferenceeffects are used to determine characteristics of the underlying featuresof the sample. Interference effects occur when the optical path lengthsof the reference and sample arms are equal within the coherence lengthof the light source. As the path length difference between the referencearm and the sample arm is changed the depth of penetration in the sampleis modified in a similar manner. Typically in biological tissues NIRlight of around 1300 nm can penetrate about 3-4 mm as is the case withdental tissue. In a time domain OCT system the reference arm delay pathrelative to the sample arm delay path is alternately increasedmonotonically and decreased monotonically to create depth scans at ahigh rate. To create a 2-dimensional scan the sample measurementlocation is changed in a linear manner during repetitive depth scanssuch as with a galvanometer.

In an OCT system 80 shown in FIG. 13, light from a low-coherence lightsource 160, such as an LED or other light source, can be used. Thislight is split and made to travel down two different optical paths beinga reference arm 164 with a built in reference delay scanner toalternately change its path length and a sample arm 76 that goes to thetooth by a beamsplitting and combining element 162. Reflected light fromboth reference and sample arms is then recombined by beamsplitting andcombining element 162, and interference effects are used to determinecharacteristics of the underlying features of the sample. The referencearm and sample arm pathlengths are made to be the same at some part ofthe reference delay scanning operation to enable observation ofinterference effects. The recombined and interfering light frombeamsplitting and combining element 162 is then sent to a detector andprocessing electronics 166 where the optical interference signal isconverted to an electrical signal which is then acquired by dataacquisition hardware and computer system 168 for further processing anddisplay. The optical elements of OCT system 80 are configured as aninterferometer.

Still referring to FIG. 13, there is shown an embodiment of imagingapparatus 10 using both FIRE imaging methods and OCT imaging. Lightsource 12, polarizing beamsplitter 18, field lens 24, a turning mirror82, imaging lens 66, and sensor 68 provide the FIRE imaging functionalong an optical path as described previously. An OCT imager 70 directslight for OCT scanning into the optical path that is shared with theFIRE imaging components. Light from OCT system 80 is directed through asample arm 76 and through a collimating lens 74 to a scanning element72, such as a galvanometer, for example. A dichroic mirror 78 istransmissive to visible light and reflective for near-IR and longerwavelengths. This sample arm light is then directed from dichroic mirror78 to tooth 20 through the optical system that includes a scanning lens84 and field lens 24. Field lens 24 is not required for non-telecentricOCT scanning. Returned light from tooth 20 travels the same optical pathand is recombined with light from the reference arm of interferometer162.

OCT scans are 2-dimensional in the plane of the impinging beam.Image-forming logic combines adjacent lines of successive 2-dimensionalscans (length along the galvanometer scan line and depth) to form amulti-dimensional volume image of the sample (tooth) structure,including portions of the tooth that lie beneath the surface.

For OCT imager 70, the light provided is continuous wave low coherenceor broadband light, and may be from a source such as a super luminescentdiode, diode-pumped solid-state crystal source, or diode-pumped rareearth-doped fiber source, for example. In one embodiment, near-IR lightis used, such as light having wavelengths near 1310 nm, for example.

While the OCT scan is a particularly powerful tool for helping to showthe condition of the tooth beneath the surface, it can be appreciatedthat this type of detailed information is not needed for every tooth orfor every point along a tooth surface. Instead, it would be advantageousto be able to identify specific areas of interest and apply OCT imagingto just those areas. Referring to FIG. 14A, there is shown a display oftooth 20. An area of interest 90 can be identified by a diagnosticianfor OCT scanning. For example, using operator interface tools atprocessing apparatus 38 and display 40 (FIGS. 1-3), an operator canoutline area of interest 90 on display 40. This could be done using acomputer mouse or some type of electronic stylus as a pointer, forexample. The OCT scan can then be performed when a probe or otherportion of imaging apparatus 10 of FIG. 13 is brought into the proximityof area of interest 90. Referring to FIG. 14B, there is shown a typicalimage from OCT data 92 in one embodiment.

Probe Embodiment

The components of imaging apparatus 10 of the present invention can bepackaged in a number of ways, including compact arrangements that aredesigned for ease of handling by the examining dentist or technician.Referring to FIG. 15, there is shown an embodiment of a hand-held dentalimaging apparatus 100 according to the present invention. Here, a handle102, shown in phantom outline, houses light source 12, sensor 68, andtheir supporting illumination and imaging path components. A probe 104attaches to a handle 102 and may act merely as a cover or, in otherembodiments, support lens 22 and turning mirror 46 in proper positioningfor tooth imaging. Control circuitry 110 can include switches, memory,and control logic for controlling device operation. In one embodiment,control circuitry 110 can simply include one or more switches forcontrolling components, such as an on/off switch for light source 12.Control circuitry 110 can be a microprocessor in the probe or externallyconnected, configured with programmed logic for controlling probefunctions and obtaining image data. Optionally, the function of controlcircuitry 110 can be performed at processing apparatus 38 (FIGS. 1-3).In other embodiments, control circuitry 110 can include sensing,storage, and more complex control logic components for managing theoperation of hand-held imaging apparatus 100. Control circuitry 110 canconnect to a wireless interface 136 for connection with a communicatingdevice, such as computer workstation or server, for example. FIG. 18shows an imaging system 150 using wireless transmission. Hand-heldimaging apparatus 100 obtains an image upon operator instruction, suchas with the press of a control button, for example. The image can thenbe sent to a control logic processor 140, such as a computerworkstation, server, or dedicated microprocessor based system, forexample. A display 142 can then be used to display the image obtained.Wireless connection of hand-held imaging apparatus 100 can beadvantageous, allowing imaging data to be obtained at processingapparatus 38 without the need for hardwired connection. Any of a numberof wireless interface protocols could be used, such as Bluetooth datatransmission, as one example.

Dental imaging apparatus 100 may be configured differently for differentpatients, such as having an adult size and a children's size, forexample. In one embodiment, removable probe 104 is provided in differentsizes for this purpose. Alternately, probe 104 could be differentlyconfigured for the type of tooth or angle used, for example. Probe 104could be disposable or could be provided with sterilizable contactcomponents. Probe 104 could also be adapted for different types ofimaging. In one embodiment, changing probe 104 allows use of differentoptical components, so that a wider angle imaging probe can be used forsome types of imaging and a smaller area imaging probe used for singletooth caries detection. One or more external lenses could be added orattached to probe 104 for specific imaging types.

Probe 104 could also serve as a device for drying tooth 20 to improveimaging. In particular, fluorescence imaging benefits from having a drytooth surface. In one embodiment, as shown in FIG. 15, a tube 106providing an outlet for directing pressurized air or other drying gasfrom a pressurized gas source 81 onto tooth 20 is provided as part ofprobe 104. Probe 104 could serve as an air tunnel or conduit forpressurized air; optionally, separate tubing could be required for thispurpose.

FIG. 16 shows an embodiment of hand-held imaging apparatus 100 having adisplay 112. Display 112 could be, for example, a liquid crystal (LC) ororganic light emitting diode (OLED) display that is coupled to handle102 as shown. A displayed image 108 could be provided for assisting thedentist or technician in positioning probe 104 appropriately againsttooth 20. Using this arrangement, a white light source is used toprovide the image on display 112 and remains on unless FIRE imaging istaking place. At an operator command entry, such as pressing a switch onhand-held imaging apparatus 100 or pressing a keyboard key, the whitelight image is taken. Then the white light goes off and the fluorescenceimaging light source, for example, a blue LED, is activated. Once thefluorescence and white light images are obtained, the white light goesback on. When using display 112 or a conventional video monitor, thewhite light image helps as a navigation aid. Using a display monitor,the use of white light imaging allows the display of an individual areato the patient.

In order to obtain an image, probe 104 can be held in position againstthe tooth, using the tooth surface as a positional reference forimaging. This provides a stable imaging arrangement and fixed opticalworking distance. This configuration yields improved image quality andconsistency. Placing probe 104 directly against the tooth has particularadvantages for OCT imaging, as described earlier, since this techniqueoperates over a small distance along the axis.

In order to image different surfaces of the tooth, a folding mirrorinside the probe, such as folding mirror 18 as shown in FIG. 22, istypically required. One problem related to this folding mirror is theundesired fogging of the mirror surface that often can occur. A numberof methods are used in intraoral cameras to address this foggingproblem. For example, in one embodiment, the mirror is heated so thatits temperature approximates the temperature of the mouth. One drawbackwith this approach is that it requires an added heating element andcurrent source for the heating element. In another embodiment of thepresent invention, an anti-fog coating is applied as a treatment to themirror surface. With this arrangement, no additional components arerequired. Another embodiment is to bond an anti-fog film to the mirrorsurface.

The embodiments shown in FIGS. 11 and 12A-12C use LEDs as light sources12 a, 12 b to illuminate tooth 20 directly, without any light-shapingoptical element. Since the divergent angle of the light from LED isusually large, a sizable portion of the emitted light strikes the innersurface of the probe, as shown in FIG. 22. The large angle rays 240 a,240 b and 240 c in FIG. 22 hit the inner surface of the probe. If theprobe inner surface is designed to be absorptive, the light hitting thesurface is absorbed and does not reach tooth 20. In one embodiment, theinner surface of the probe is reflective, so that the light incident onthis surface is reflected and eventually reaches the tooth. There aretwo advantages with this design. One advantage is increased efficiency,since all of the light reaches tooth 20 except for some absorption loss.Another advantage relates to the uniformity of the illumination on tooth20. With a reflective inner surface, the probe operates as a light pipe.This integrates the light spatially and angularly, and provides uniformillumination to the tooth.

Imaging Software

One method for reducing false-positive readings or, similarly,false-negative readings, is to correlate images obtained from multiplesources. For example, images separately obtained using x-ray equipmentcan be combined with images that have been obtained using imagingapparatus 10 of the present invention. Imaging software, provided inprocessing apparatus 38 (FIGS. 1-3) allows correlation of images oftooth 20 from different sources, whether obtained solely using imagingapparatus 10 or obtained from some combination of devices includingimaging apparatus 10.

Referring to FIG. 17, there is shown, in block diagram form, aprocessing scheme using images from multiple sources. A fluorescenceimage 120, a reflectance image 122, and a white light image 124 areobtained from imaging apparatus 10, as described earlier. An x-ray image130 is obtained from a separate x-ray apparatus. Image correlationsoftware 132 takes two or more of these images and correlates the dataaccordingly to form a composite image 134 from these multiple imagetypes. Composite image 134 can then be displayed or used by automateddiagnosis software in order to identify regions of interest for aspecific tooth. In one embodiment, the images are provided upon operatorrequest. The operator specifies a tooth by number and, optionally,indicates the types of image needed or the sources of images to combine.Software in processing apparatus 38 then generates and displays theresultant image.

As one example of the value of using combined images, white light image124 is particularly useful for identifying stained areas, amalgams, andother tooth conditions and treatments that might otherwise appear toindicate a caries condition. However, as was described earlier, the useof white light illumination is often not sufficient for accuratediagnosis of caries, particularly in its earlier stages. Combining thewhite light image with some combination that includes one or more offluorescence and x-ray images helps to provide useful information ontooth condition. Similarly, any two or more of the four types of imagesshown in FIG. 17 could be combined by image correlation software 132 forproviding a more accurate diagnostic image.

Imaging software can also be used to help minimize or eliminate theeffects of specular reflection. Even where polarized light componentscan provide some measure of isolation from specular reflection, it canbe advantageous to eliminate any remaining specular effects using imageprocessing. Data filtering can be used to correct for unwanted specularreflection in the data. Information from other types of imaging can alsobe used, as is shown in FIG. 17. Another method for compensating forspecular reflection is to obtain successive images of the same tooth atdifferent light intensity levels, since the relative amount of specularlight detected would increase at a rate different from light due toother effects.

Another key feature of the image processing software is to enhance theimage obtained and automatically highlight suspicious areas such aswhite spots. FIG. 20 shows the flowchart for image processing workflow.As the first step 202, the software reads white light and fluorescenceimages. The software then analyzes the contents in different colorplanes in white light and fluorescence images in a step 204. With theinformation obtained from the white light and fluorescence images,different image processing algorithms such as color rendering, contrastenhancement, and segmentation, can be applied to enhance the image insteps 206 and 208. Some of the algorithms are discussed earlier withrelation to image processing. Also, image processing algorithms can beused to identify the nature of each region, based on the colorinformation in each color plane, and to highlight each regionautomatically in an enhancement step 210. As a final step 212, the toothinformation, such as the size, shape and status of the suspicious area,can be extracted and displayed to the dental professionals.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention as described above, and as noted in the appended claims, by aperson of ordinary skill in the art without departing from the scope ofthe invention.

For example, various types of light sources 12 could be used, withvarious different embodiments employing a camera or other type of imagesensor. While a single light source 12 could be used for fluorescenceexcitation, it may be beneficial to apply light from multiple incidentlight sources 12 for obtaining multiple images. Referring to thealternate embodiment of FIG. 8, light source 12 might be a more complexassembly that includes one light source 16 a for providing light ofappropriate energy level and wavelength for exciting fluorescentemission and another light source 16 b for providing illumination atdifferent times. The additional light source 16 b could provide light atwavelength and energy levels best suited for back-scattered reflectanceimaging. Or, it could provide white light illumination, or othermulticolor illumination, for capturing a white light image or multicolorimage which, when displayed side-by-side with a FIRE image, can help toidentify features that might otherwise confound caries detection, suchas stains or hypo calcification. The white light image itself might alsoprovide the back-scattered reflectance data that is used with thefluorescence data for generating the FIRE image. Supporting optics forboth illumination and image-bearing light paths could have any number offorms. A variety of support components could be fitted about the toothand used by the dentist or dental technician who obtains the images.Such components might be used, for example, to appropriately positionthe light source or sensing elements or to ease patient discomfortduring imaging.

Thus, what is provided is an apparatus and method for caries detectionat early and at later stages using combined effects of back-scatteredreflectance and fluorescence.

PARTS LIST

-   10 imaging apparatus-   12 light source-   12 a light source-   12 b light source-   13 diffuser-   14 lens-   15 filter-   16 a light source-   16 b light source-   17 filter-   18 polarizing beamsplitter-   20 tooth-   22 lens-   24 field lens-   26 filter-   28 filter-   30 camera-   32 camera-   34 beamsplitter-   38 processing apparatus-   40 display-   42 polarizer-   42 a polarizer-   42 b polarizer-   44 analyzer-   46 turning mirror-   48 dichroic mirror-   49 a fiber bundle-   49 b fiber bundle-   49 c fiber bundle-   49 d fiber bundle-   50 fluorescence image-   52 reflectance image-   54 white-light image-   56 filter-   58 carious region-   60 FIRE image-   62 threshold image-   64 enhanced threshold FIRE image-   66 lens-   68 sensor-   68 a sensor-   68 b sensor-   70 OCT imager-   72 scanning element-   74 lens-   76 sample arm-   78 dichroic mirror-   80 OCT system-   81 gas source-   82 mirror-   83 switch-   84 scanning lens-   90 area of interest-   92 OCT data-   96 filter curve-   98 white light curve-   100 imaging apparatus-   102 handle-   104 probe-   106 tube-   108 image-   110 control circuitry-   112 display-   120 fluorescence image-   122 reflectance image-   124 white light image-   130 x-ray image-   132 image correlation software-   134 composite image-   136 wireless interface-   140 control logic processor-   142 display-   150 imaging system-   160 light source-   162 beamsplitting and combining element-   164 reference arm-   166 detector and processing electronics-   168 data acquisition hardware and computer system-   202 step-   204 step-   206 step-   208 step-   210 step-   212 step-   220 object (tooth)-   222 lens 1-   224 intermediate image-   226 lens 2-   228 final image-   240 a ray-   240 b ray-   240 c ray-   250 a light source-   250 b light source-   252 a image-   252 b image-   254 crosshair-   256 cross point

The invention claimed is:
 1. An apparatus for imaging a toothcomprising: at least one light source adapted to provide, along anoptical path, an incident light to illuminate the tooth, the incidentlight having a first spectral band for obtaining a back-scatteredreflectance image of the tooth (20) and a second spectral band forexciting a fluorescence image of the tooth; a spectral separator in theoptical path to direct light of the first spectral band that isreflected from the tooth along a first optical return path between thespectral separator and a first sensor, the first sensor to obtain theback-scattered reflectance image of the tooth and to direct second lightfrom the tooth along a different second optical return path between thespectral separator and a second sensor, the second sensor to obtain thefluorescence image of the tooth; an analyzer disposed in the firstoptical return path to direct light of the first spectral band andhaving a prescribed polarization state to the first sensor; and a lenspositioned in the optical path to form an image of the tooth, throughthe analyzer, onto the first sensor for obtaining the back-scatteredreflectance image of the tooth.
 2. The apparatus of claim 1 wherein thespectral separator is a dichroic beamsplitter.
 3. The apparatusaccording to claim 1 further comprising an optical coherence tomography(OCT) system coupled to the optical path to obtain an OCT image of aportion of the tooth using optical coherence tomography in addition tothe fluorescence image and the back-scattered reflectance image.
 4. Theapparatus according to claim 1 further comprising an optical coherencetomography (OCT) system directing light for OCT scanning into theoptical path.
 5. The apparatus according to claim 4 where the opticalcoherence tomography (OCT) system comprises: a low-coherence lightsource; a beamsplitting and combining element to split and guidereceived light from the low-coherence light source down two differentoptical paths being a reference arm optical path with a built inreference delay scanner to alternately change its path length and asample arm optical path that goes to the tooth, where path lengths ofthe reference arm optical path and the sample arm are made to be thesame, where reflected light from both reference arm optical path and thesample arm optical path is then recombined by the beamsplitting andcombining element; a detector to receive an optical interference signalcaused by the recombined and interfering light from the beamsplittingand combining element; a data acquisition hardware and computer systemto receive an electrical signal from the detector generated by theoptical interference signal; and an OCT imager to direct light in thesample arm optical path for OCT scanning to the tooth through acollimating lens to a scanning element to a dichroic mirror transmissiveto visible light and reflective for near-IR and longer wavelengths. 6.The apparatus of claim 1 where the incident light to illuminate thetooth includes a first polarization state, and where the prescribedpolarization state is orthogonal to the first polarization state.
 7. Theapparatus of claim 1, where the at least one light source comprises twolight sources providing incident light along respective paths to thetooth, the first light source being adapted to provide incident lighthaving the first spectral band for obtaining the back-scatteredreflectance image of the tooth and the second light source being adaptedto provide the second spectral band for exciting the fluorescence imageof the tooth.
 8. The apparatus of claim 1, further comprising apolarizer in the path of the incident light to illuminate the tooth anddisposed to direct light having a first polarization state toward thetooth.